1. Formerly University of Alberta, Institute of Geophysical Research, Department of Physics, Edmonton, Alberta, Canada; presently University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, Alberta, Canada.(corresponding author).
2. Formerly University of Alberta, Institute of Geophysical Research, Department of Physics, Edmonton, Alberta, Canada; presently Purdue University, Earth, Atmospheric, and Planetary Sciences Department, West Lafayette, Indiana, USA..